Honeycomb ceramics structure body and method for producing the same

ABSTRACT

A ceramics structure body having chemical composition of 42 to 56 wt % of SiO 2 , 30 to 45 wt % of Al 2 O 3  and 12 to 16 wt % of MgO, crystalline phase mainly composed of cordierite, a porosity of 55 to 65%, an average pore size of 15 to 30 μm; and the total area of pores exposed on surfaces of partition walls constituting the honeycomb ceramics structure body being 35% or more of the total area of partition wall surfaces. Fifteen to 25 wt % of graphite and 5 to 15 wt % of a synthetic resin are added as a pore forming agent to a cordierite-forming raw material; the resultant is kneaded and molded into a honeycomb shape; and the resultant is dried and fired to produce above-mentioned honeycomb ceramics structure body. According to this honeycomb ceramics structure body, a low pressure loss and a high collection efficiency can be attained.

TECHNICAL FIELD

The present invention relates to a honeycomb ceramics structure bodywhich, for example, can attain a high collection efficiency with a lowpressure loss and can be suitably used as a diesel particulate filter(DPF), and a method for producing the same.

BACKGROUND ART

In these years, the diesel particulate filters (DPFs) for collectingparticulates discharged from diesel engines have been attractingattention, which are required to attain high collection efficiency withlow pressure loss.

As DPFs, the honeycomb structure bodies made of cordierite have beenconventionally used; in pursuit of such high collection efficiency withlow pressure loss as described above, the improvement has hitherto beenmade in the honeycomb structure bodies as to the porosity and poredistribution thereof.

JP-A-9-77573 discloses a honeycomb structure having a specified the poredistribution on the surface of the partition walls with an enlargedporosity and an enlarged average pore size. JP-A-11-333293 describeshoneycomb structure body having an enlarged porosity in addition to thinpartition walls of a prescribed value or less.

In addition, JP-B-7-38930 discloses the production method for ahoneycomb structure body having a high porosity by using acordierite-forming raw material containing a talc powder and a silicapowder each composed of coarser particles of a prescribed particle sizeor more. Japanese Patent No. 2726616 discloses a honeycomb structurebody having a specified pore distribution and surface roughness inaddition to a high porosity.

In the above described prior art, in order to increase the porosity, acordierite-forming raw material is pulverized into coarse particles,graphite, wood powder, and a foaming agent are added as pore formingagents, or the like, but sufficiently satisfactory effects have not yetbeen obtained.

More specifically, when a cordierite-forming raw material is pulverizedinto coarse particles, the cordierite-forming reaction does not proceedto a sufficient extent, so that it is difficult to attain a low thermalexpansion. When graphite is used as a pore forming agent, the followingproblems occur: the dielectric constant of a formed body with additionof graphite is decreased, so that it becomes difficult to perform auniform drying of the formed body by the dielectric drying or themicrowave drying with increase in addition amount of graphite.Furthermore, the firing period at the range from 800 to 1000° C. is tobe so elongated in the firing process that it is necessary to suppressthe rapid combustion of the graphite.

Moreover, when the starches or wood powders are used as the pore formingagent, it is necessary to add a large amount of water in order to makethe body for ceramics attain a prescribed hardness in the kneadingprocess, so that the efficiency in the drying process becomes poor; andin the firing process the starches and wood powders rapidly burn between200 and 400° C. to release a large amount of heat, so that it isdifficult to prevent the firing cracking. As above, in the prior art, ithas been extremely difficult to increase the porosity beyond aprescribed value.

DISCLOSURE OF THE INVENTION

As a result of a diligent investigation performed in view of the abovedescribed problems in the prior art, the present inventors reached thepresent invention based on the findings that a very low pressure lossand a high collection efficiency can be attained when the porosity ofthe honeycomb structure body is increased to a prescribed value or more,and the total sum of the areas of the pores exposed on the partitionwall surfaces is made to a prescribed value or more, with payingattention to the importance of the pore area on the partition wallsurfaces with which surfaces the exhaust gas actually comes into contactand through which surfaces the exhaust gas passes.

In other words, according to the present invention, there is provided ahoneycomb ceramics structure body having chemical composition of 42 to56 wt % of SiO₂, 30 to 45 wt % of Al₂O₃ and 12 to 16 wt % of MgO, andthe crystalline phase mainly composed of cordierite, characterized inthat said honeycomb ceramics structure body has a porosity of 55 to 65%,an average pore size of 15 to 30 μm; and the total area of the poresexposed on the surfaces of the partition walls constituting thehoneycomb ceramics structure body being 35% or more of the total area ofthe partition wall surfaces.

In the honeycomb ceramics structure body of the present invention, it ispreferable that the total area of the pores exposed on the partitionwall surfaces is 40% or more of the total area of the partition wallsurfaces, and the average pore size is from 15 to 25 μm. Furthermore, itis preferable that the partition wall thickness is 300 μm or less. Inaddition, the permeability preferably is from 1.5 to 6 μm². It is alsopreferable that the coefficient of thermal expansion of the honeycombceramics structure body of the present invention between 40 and 800° C.is 0.5×10⁻⁶/° C. or less.

The honeycomb ceramics structure body of the present invention can besuitably used as a diesel particulate filter (DPF) collecting theparticulates discharged form a diesel engine.

In addition, according to the present invention, there is provided amethod for producing a honeycomb ceramics structure body having chemicalcomposition of 42 to 56 wt % of SiO₂, 30 to 45 wt % of Al₂O₃ and 12 to16 wt % of MgO, the crystalline phase mainly composed of cordierite, aporosity of 55 to 65%, an average pore size of 15 to 30 μm; and thetotal area of the pores exposed on the surfaces of the partition wallsconstituting the honeycomb ceramics structure body being 35% or more ofthe total area of the partition wall surfaces, characterized in that 15to 25 wt % of graphite and 5 to 15 wt % of a synthetic resin are addedas a pore forming agent to a cordierite-forming raw material, theresultant is kneaded and molded into a honeycomb shape, and thus formedbody is dried and fired to produce a honeycomb ceramics structure body.

In the above description, the synthetic resin is preferably any one ofpoly(ethylene terephthalate) (PET), poly(methyl methacrylate) (PMMA),and phenolic resin, or a combination thereof, and the average particlesize of the raw material talc in the cordierite-forming raw material ispreferably 50 μm or less and the average particle size of the rawmaterial silica is 60 μm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a SEM photograph of the rib section in the honeycombceramics structure body of Example 1.

FIG. 2 shows a SEM photograph of the surface of the partition wall(membrane surface) in the honeycomb ceramics structure body of Example1.

FIG. 3 shows a SEM photograph of the rib section in the honeycombceramics structure body of Example 5.

FIG. 4 shows a SEM photograph of the surface of the partition wall(membrane surface) in the honeycomb ceramics structure body of Example5.

FIG. 5 is a graph showing the relationships between percent weightreductions (TG) and heat flows (DTA) of the samples from ComparativeExample 7 and Example 7.

FIG. 6 is a graph showing the relationship between the soot depositiontime and the pressure loss.

BEST MODE FOR CARRYING OUT THE INVENTION

The honeycomb ceramics structure body of the present invention has thechemical composition 42 to 56 wt % of SiO₂, 30 to 45 wt % of Al₂O₃, and12 to 16 wt % of MgO, the crystalline phase mainly composed ofcordierite, the porosity of 55 to 65%, the average pore size of 15 to 30μm, and the total area of the pores exposed on the surface of thepartition wall constituting the honeycomb ceramics structure body being35% or more of the total area of the partition wall surface.

In the honeycomb ceramics structure body of the present invention, theporosity ranges from 55 to 65%. With the porosity below than 55%, thepressure loss of the exhaust gas is unpreferably increased, while withthe porosity exceeding 65%, the mechanical strength of the honeycombstructure body is so remarkably degraded that the honeycomb structurebody cannot be endurable to the actual use.

In addition, in this honeycomb ceramics structure body, the average poresize is 15 to 30 μm, and preferably from 15 to 25 μm. With the averagepore size below than 15 μm, the collection efficiency is increased, butthe pressure loss unpreferably becomes high. On the other hand, with theaverage pore size exceeding 30 μm, the pressure loss is satisfactorilylow, but there occurs an increase in the probability that theparticulates in the exhaust gas are not collected since they passthrough the larger pores. In particular, when the wall thickness of thepartition wall of the honeycomb ceramics structure body is 300 μm orless, the degradation of the collection efficiency becomes remarkable.In addition, when the average pore size exceeds 30 μm and the porosityis below 55%, the initial pressure loss is low, but with increasing timeof use the pressure loss tends to increase sharply. It is consideredthat the particulates in the exhaust gas tend to be deposited in theinterior of the partition wall by passing through the large pores, andthere is caused an increase in the possibility that the depositedparticulates remain unburned when renewed by combustion. In addition, itis also considered that even with a continuous renewal type honeycombbody supporting an oxidation catalyst on the partition wall surface, theparticulates similarly remain unburned and are deposited in the interiorof the partition wall to increase the pressure loss. Accordingly, it ismore preferable that the average pore size falls within the range from15 to 25 μm.

In addition, in the present invention, the total area of the poresexposed on the surface of the partition wall constituting the honeycombceramics structure body is 35% or more of the total area of thepartition wall surface. In such a manner, by making the total area ofthe pores exposed on the surface of the partition wall be a prescribedvalue or more, a high collection efficiency can be attained with a verylow pressure loss in relation to the exhaust gas. Incidentally, it ispreferred that the total area of the pores exposed on the partition wallsurface is 40% or more of the total area of the partition wall surface,and that it is 60% or less.

In addition, in the honeycomb ceramics structure body of the presentinvention, the permeability can be made to range from 1.5 to 6 μm². Ahoneycomb structure body having the permeability of this range canattain a high collection efficiency with a low pressure loss in relationto the exhaust gas.

Here, the permeability in the present specification means a numericalvalue obtained by the following formula 1:$C = {\frac{8{FTV}}{\pi \quad {{D^{2}\left( {P^{2} - 13.839^{2}} \right)}/13.839} \times 68947.6} \times 10^{8}}$

“In the above formula, C denotes the permeability (μm²), F the gas flowrate (cm³/s), T the sample thickness (cm), V the gas viscosity(dynes·s/cm²), D the sample diameter (cm), and P the gas pressure (PSI).In addition, as for the numerical values in the above formula, thefollowing relations hold: 13.839 (PSI)=1 (atm), and 68947.6(dynes/cm²)=1 (PSI).”

In the honeycomb ceramics structure body of the present invention, thecoefficient of thermal expansion between 40 and 800° C. can be made to0.5×10⁻⁶/° C. or less. With such a coefficient of thermal expansion, anexcellent thermal shock resistance is exhibited, so that the honeycombstructure body will be hardly damaged even when sharp temperaturechanges repeatedly occur.

In addition, since as described above the honeycomb ceramics structurebody of the present invention is high in collection efficiency, it canbe suitably applied to a thin-wall honeycomb structure body having sucha partition wall thickness of 300 μm or less.

Accordingly, the honeycomb ceramics structure body of the presentinvention having the above described constitution can be very preferablyapplied as a diesel particulate filter (DPF) collecting the particulatesdischarged from a diesel engine.

Then, description will be made on the method for producing a honeycombceramics structure body according to the present invention.

The honeycomb ceramics structure body according to the present inventioncan be produced through the following sequence of processes: firstly, acordierite-forming raw material is prepared in which talc, kaoline,calcined kaoline, alumina, aluminum hydroxide, and silica are blended insuch prescribed ratios that the chemical composition is within a rangecapable of containing 42 to 56 wt % of SiO₂, 30 to 45 wt % of Al₂O₃, and12 to 16 wt % of MgO; 15 to 25 wt % of graphite and 5 to 15 wt % of asynthetic resin such as PET, PMMA, and phenolic resin are added as apore forming agent, and methylcelluloses and a surfactant are added inthe prescribed amounts to the raw material, and subsequently appropriateamount of water is added; the resultant mixture is kneaded to form abody for ceramics. Then, the body for ceramics is subjected to vacuumdegassing, subsequently extruded into a honeycomb structure, dried bydielectric drying, microwave drying, or hot air drying, and subsequentlyfired within a temperature of 1400 to 1435° C. as a highest temperature,to produce the honeycomb ceramic structure body of the presentinvention.

In addition, the staggered pattern clogging of the end surfaces in thehoneycomb ceramics structure body is performed after the drying process,or after the firing process where the honeycomb structure body is firedagain.

The production method of the present invention is characterized in that,to the cordierite-forming raw material, 15 to 25 wt % of graphite isadded as a pore forming agent, and simultaneously 5 to 15 wt % of asynthetic resin such as PET, PMMA, or phenolic resin all of which is lowin heat flow during combustion are added. By doing so, it has becomepossible to produce, inexpensively and in a large scale, a cordieritehoneycomb structure body having a porosity of 55% or more.

With the addition of graphite exceeding 25 wt % in relation to thecordierite-forming raw material, it is difficult to perform a uniformdrying by the dielectric drying or the microwave drying, andsimultaneously it is required that in the firing process, the combustiontime within the range from 800 to 1000° C., in which range graphite iscombusted, is required to be made longer so as to suppress the rapidcombustion of the graphite. When the temperature rising rate in thecombustion range of graphite is too large, graphite is combusted rapidlyto result in a wide temperature distribution in the honeycomb structurebody, involving a risk of generating cracks. In addition, when graphiteremains unburned, it affects adversely the cordierite-forming reactionto be performed in a high temperature of 1200° C. or above, involving arisk of increasing the thermal expansion. Thus, in view of theindustrial large-scale production, the addition amount of graphite isrequired to be 25 wt % or less, and is more preferably 20 wt % or less.The lower limit for the addition amount of graphite is required to be 15wt % or more in view of the pore forming property and heat flow.

In the present invention, by adding a prescribed amount of a syntheticresin relatively low in heat flow during combustion to the graphite, itbecomes possible to produce a honeycomb structure body having such alarge porosity as is 55% or more.

In addition, in order to enlarge the total area of the pores exposed onthe surface of the partition wall of a honeycomb structure body, as inthe honeycomb ceramics structure body of the present invention, it isnecessary to increase the porosity and simultaneously it is alsonecessary to control the pores formed by talc and silica in the processof the cordierite-forming reaction. When the talc raw material or thesilica raw material is made to be coarse particles, it is possible tomake the average pore size be larger, but the formed pores do notnecessarily appear on the partition wall surfaces, and it results in theformation of coarse pores in the interior of the partition walls. Thisis because the coarse particles tend to gather together, duringextrusion, in the central part of the partition wall.

Thus, in the present invention, it becomes possible to form poreseffectively on the surface of the partition walls by controlling theaverage particle size of the talc raw material to 50 μm or less and theaverage particle size of the silica raw material to 60 μm or less, bothmaterial being important for forming pores; consequently, the ratio ofthe total area of the pores exposed on the surfaces of the partitionwalls in a honeycomb structure body to the total area of the partitionwall surfaces can be made to be 35% or more. Moreover, it is morepreferable that the average particle size of the talc raw material iswithin the range of from 20 to 50 μm and the average particle size ofthe silica raw material is within the range of from 20 to 60 μm.

Description will be made below on the present invention on the basis ofthe specific Examples, but the present invention is not limited to theseExamples.

EXAMPLES 1 TO 11 AND COMPARATIVE EXAMPLES 1 TO 16

The cordierite-forming raw materials and pore forming agents shown inTable 1 were blended in the respective content ratios shown in Table 2;to the mixtures thus obtained methylcellulose and hydroxypropoxylmethylcellulose were added by 2 wt %, respectively, then a fatty acidsoap was added as a surfactant by 0.5 wt %, and further an appropriateamount of water was added, to form respective puddles. Then, using thesepuddles, as shown in Table 3, a series of honeycomb structure bodies ofφ150 mm×150 mm (length) were extruded, where the cell structure was suchthat either the wall thickness was 300 μm and the number of the cells 31cells/cm², or the wall thickness was 430 μm and the number of the cells16 cells/cm². The respective extruded bodies were subjected to thedielectric drying and hot air drying to remove water. Then, the formedbody was fired under the conditions that the highest temperature was1415° C. and the retention time at the highest temperature was 8 hours;both end faces were clogged alternately in a staggered pattern with aslurry-like cordierite-forming raw material, then the respective formedbodies were again fired with the highest temperature of 1420° C., andthus respective honeycomb ceramics structure bodies as an evaluationsample were produced.

The physical properties and evaluation results for the honeycombstructure bodies thus obtained are shown in Table 3.

TABLE 1 Average grain Chemical analysis (%) Raw material size (μm)IgLoss SiO₂ Al₂O₃ Fe₂O₃ TiO₂ MgO CaO + Na₂O + K₂O Main raw material TalcA 25 5.5 62 0.15 1.75 0.005 31 0.15 Talc B 45 5 63 0.1 0.02 0 31.5 0.5Talc C 25 6.5 59.5 0.7 2.5 0.02 30.5 0.02 Talc D 35 4.8 62.5 0.2 0.40.01 31.7 0.1 Talc E 55 5 63 0.1 0.02 0 31.5 0.5 Calcined talc 25 0 66.30.1 0.0 0.0 33.2 0.5 Kacline A 9 14 45.5 39 0.2 0.7 0.01 0.09 Kaoline B4 13.5 46 39 0.4 0.9 0.02 0.1 Kaoline C 5 14 45.5 39 0.3 0.7 0.01 0.1Alumina 6 0.05 0.02 99.5 0.02 0 0 0.2 Aluminum hydroxide A 1 34 0 65.5 00 0 0.35 Aluminum hydroxide B 2 34 0.05 65.5 0 0 0 0.3 Silica A 20 0.199.8 0.02 0.02 0 0 0.02 Silica B 110 0.1 99.7 0.1 0 0 0 0.01 Silica C 400.1 99.8 0.1 0.02 0 0 0.01 Silica D 50 0.1 99.8 0.02 0.02 0 0 0.02 Fusedsilica 42 0.1 99.8 0.1 0.02 0 0 0.01 Pore forming agent Graphite 40 99.50.2 PET 60 99.9 PMMA 60 99.9 Phenolic resin 60 99.8 Cornstarch 60 99.8Walnut powder 150 99.8

TABLE 2 Average grain size of raw material (μm) Blending composition ofcordierite-forming raw material (wt %) Silica Batch Calcined Kao-Calcined Alu- Aluminum Fused Talc raw raw No. Talc talc line kaolinemina hydroxide Silica silica Pore forming agent (wt %) material material 1 B: 28 10 B: 14 10 11.5 A: 16.5 C: 10 0 Graphite: 20 43 40  2 A:B(1:1) 41 0 B: 16 0 15 B: 16 A: 12 0 Graphite: 20 35 20  3 A: 41 0 B: 160 15 B: 16 A: 12 0 Graphite: 20 25 20  4 A: B(1:1) 41 0 C: 16 0 15 B: 16B: 12 0 Graphite: 20 35 50  5 A: 41 0 B: 16 0 15 B: 16 B: D(5:7) 12 0Graphite: 20 25 75 6-1 A: 40 0 A: 17 0 15 A: 16 0 12 Graphite: 30 25 426-2 A: 40 0 A: 17 0 15 A: 16 0 12 Graphite: 20 + cornstarch: 10 25 426-3 A: 40 0 A: 17 0 15 A: 16 0 12 Graphite: 20 +walnut powder: 10 25 42 6 A: 40 0 A: 17 0 15 A: 16 0 12 Graphite: 15 + PET: 15 25 42  7 C: 39 0B: 15 0 15 B: 17 B: 3 11 Graphite: 15 + PET: 15 25 57  8 B: 39 0 B: 14 015 B: 17 B: 4 11 Graphite: 15 + PET: 15 50 60  9 A: 39 0 B: 15 0 15 B:17 B: 3 11 Graphite: 15 + PET: 15 25 57 10 B: 39 0 B: 14 0 15 B: 17 B: 411 Graphite: 15 + PET: 15 35 60 11 B: C(1:1) 39 0 B: 14 0 15 B: 17 B: 411 Graphite: 15 + PET: 15 38 60 12 C: 41 0 B: 16 0 15 A: 16 A: 12 0Graphite: 20 + PET: 10 25 20 13 C: 41 0 B: 16 0 15 A: 16 A: 12 0Graphite: 20 + PMMA: 10 25 20 14 C: 41 0 B: 16 0 15 A: 16 A: 12 0Graphite: 20 + phenolic resin: 10 25 20 15 C: 40 0 B: 17 0 15 A: 16 0 12Graphite: 20 + PET: 10 25 42 16 C: 41 0 B: 16 0 15 A: 16 A: 12 0Graphite: 20 + PET: 10 25 20 17 E: 41 0 B: 16 0 15 A: 16 A: 12 0Graphite: 20 + PET: 10 55 20 18 C: 41 0 B: 16 0 15 A: 16 B: D(1:1) 12 0Graphite: 20 + PET: 10 25 80 19 E: 41 0 B: 16 0 15 A: 16 B: D(1:1) 12 0Graphite: 20 + PET: 10 55 80 20 A: 41 0 B: 16 0 15 A: 16 A: 12 0Graphite: 20 25 20 21 A: 40 0 B: 20 0 14 A: 16 A: 10 0 Graphite: 20 2520

TABLE 3 Average Initial Collection Wall Number of cells pore sizePorosity Area pressure efficiency Permeability NO. Dough thickness (μm)(cells/cm²) CTE (μm) (%) ratio (%) loss (mmHg) (%) (μm²) ComparativeExample 1 Batch 1 300 31 0.9 25 52 25 80 90 4.1 2 Batch 2 300 31 0.5 3050 23 85 95 5.3 3 Batch 3 300 31 0.3 16 54 30 85 95 1.4 4 Batch 4 300 310.8 30 55 32 80 90 6.1 5 Batch 5 300 31 0.4 35 50 20 70 80 7.6 6 Batch6-1 300 31 Measurement was impossible because of the cracks generated inthe firing process. 7 Batch 6-2 300 31 8 Batch 6-3 300 31 Example 1Batch 6 300 31 0.4 25 63 45 65 95 4.8 2 Batch 7 300 31 0.4 28 60 41 6095 6.0 3 Batch 8 300 31 0.5 23 62 43 55 90 4.1 4 Batch 9 300 31 0.3 2860 43 65 95 5.9 5 Batch 10 300 31 0.3 23 62 43 60 95 4.0 6 Batch 11 30031 0.4 26 62 42 65 90 5.3 7 Batch 12 300 31 0.3 17 55 35 75 95 1.9 8Batch 13 300 31 0.5 15 56 36 70 95 1.5 9 Batch 14 300 31 0.3 17 57 39 7095 2.1 10 Batch 15 300 31 0.3 20 60 40 65 90 3.0 11 Batch 16 300 31 0.320 58 38 65 90 2.9 Comparative Example 9 Batch 17 300 31 0.6 32 56 34 6085 7.0 10 Batch 18 300 31 0.7 35 55 33 60 80 8.3 11 Batch 19 300 31 0.938 53 29 50 70 9.6 12 Batch 20 300 31 0.3 14 54 26 85 95 1.3 13 Batch 21300 31 0.2 10 53 22 95 95 1.0 14 Batch 12 430 16 0.3 17 55 35 95 98 2.015 Batch 13 430 16 0.4 15 56 36 90 98 1.6 16 Batch 15 430 16 0.3 20 6040 85 95 2.9

Here, the measurements of the average pore size, porosity, ratio of thetotal area of the pores exposed on the partition wall surfaces to thetotal area of the partition wall surfaces (area ratio), permeability,coefficients of thermal expansion between 40 and 800° C. (CTE), pressureloss, and collection efficiency of a honeycomb ceramics structure bodywere performed as follows.

The average pore size and the porosity were obtained from the poredistribution measured by the mercury intrusion method. The porosity wascalculated from the total pore volume.

Area ratio: the area ratio of the pores exposed on the partition wallsurfaces was obtained by analyzing a photograph of the partition wallsurfaces obtained by the SEM observation using an image analysisapparatus.

CTE: the measurement was made with the differential measurement methodusing a quartz standard specimen.

Permeability: a portion of the partition wall was cut out from ahoneycomb ceramics structure body, and was so processed that theconcavities and convexities were removed to prepare a test sample; thesample was so placed between a pair of members of the sample holder ofφ20 mm that no gas leaks, with one member in contact with the top faceof the test sample and the other in contact with the bottom face of thetest sample, and then a gas was made to flow into the sample holder at aspecified gas pressure; and the permeability was obtained from the gasamount which passed through the test sample on the basis of thefollowing formula 1:$C = {\frac{8{FTV}}{\pi \quad {{D^{2}\left( {P^{2} - 13.839^{2}} \right)}/13.839} \times 68947.6} \times 10^{8}}$

“In the above formula, C denotes the permeability (μm²), F the gas flowrate (cm³/s), T the sample thickness (cm), V the gas viscosity(dynes·s/cm²), D the sample diameter (cm), and P the gas pressure (PSI).In addition, as for the numerical values in the above formula, thefollowing relations hold: 13.839 (PSI)=1 (atm), and 68947.6(dynes/cm²)=1 (PSI).”

Pressure loss: soot was generated using a light oil gas burner, and aDPF was arranged at a position downstream of the burner; the combustiongas containing the soot was made to flow into the DPF at the gas flowrate of 2.4 Nm³/min and the temperature of about 150° C.; and thepressure loss was obtained from the time variation of the pressuredifference between before and after the DPF measured while the soot wasbeing deposited in the DPF.

Collection efficiency: soot was generated using a light oil gas burner,and a DPF was arranged at a position downstream of the burner; thecombustion gas containing the soot was made to flow into the DPF at thegas flow rate of 2.4 Nm³/min and the temperature of about 150° C.; andthe collection efficiency was obtained from the weight ratio between thesoot weights in the definite fractional gas flows respectively branchedfrom the gas flow respectively at some positions upstream and downstreamof the DPF.

(Discussion)

FIG. 1 shows a SEM photograph of the rib section of the honeycombceramics structure body of Example 1, and FIG. 2 shows a SEM photographof the partition wall surface (membrane surface) of the honeycombceramics structure body of Example 1.

In addition, FIG. 3 shows a SEM photograph of the rib section of thehoneycomb ceramics structure body of Comparative Example 5, and FIG. 4shows a SEM photograph of the partition wall surface (membrane surface)of the honeycomb ceramics structure body of Comparative Example 5.

In the two photographs of FIGS. 2 and 4, the white looking areas (theyellow looking areas in the photographs substituted for drawings) arethe pores exposed on the partition wall surface (surface pores). Thehigh area ratio of the surface pores leads to the decrease of theinitial pressure loss.

FIGS. 3 and 4 show the fine structure of the honeycomb structure body ofComparative Example 5.

From the photographs of FIGS. 3 and 4, it can be seen that in the ribsection of Comparative Example 5, very large pores gather together nearthe central part of the rib, owing to the use of the silica raw materialcomprising coarse particles having the average particle size of 75 μm.Large pores are known to be formed by use of coarse particle rawmaterial of talc or silica; however, the coarse raw material particlesgather together near the central part of the ribs when the honeycombstructure is formed by extrusion, and hence the large pores are formedonly in the central part of the ribs. In the photograph of the membranesurface of Comparative Example 5, the total area of the pores exposed onthe partition wall surfaces was only 20%. Comparative Example 5 is notso high in pressure loss, but the collection efficiency is as poor as80% owing to the effect ascribable to the large pores.

In Example 1 as shown in FIGS. 1 and 2, a synthetic resin of PET wasused as a pore forming agent together with graphite in order to increasethe porosity. As a result, the porosity became as high as 63%. It wasalso confirmed that the use of a synthetic resin had the effects ofincreasing the porosity of the honeycomb structure body, andsimultaneously increasing the surface pores as can be seen from theappearance of the top end face and that of the bottom end face of therib section in FIG. 1. When the photograph of FIG. 2 was subjected tothe image analysis, the area ratio of the surface pores in Example 1 wasfound to be as high as 45%, and consequently, as shown in Table 3, thepermeability was 4.8 μm², the initial pressure loss was suppressed to avery low level of 65 mmHg, and the collection efficiency reached a levelas high as 95%.

Conventionally, starch and the like have been used as the pore formingagents substituting for graphite, but when used in a large amount, thereoccurs a problem that “fissure” is generated in the drying process,firing process, and the like. FIG. 5 shows the relationship between thepercent weight reduction (TG) and heat flow (DTA) in the dough(Comparative Example 7: batch 6-2) containing starch (cornstarch) in 10wt % and graphite in 20 wt %, and that in the dough (Example 7: batch12) containing a synthetic resin (PET) in 10 wt % and graphite in 20 wt%.

As can be seen from FIG. 5, when starch was used as a pore formingagent, the starch was thermally decomposed around the temperature rangefrom 300° C. to 350° C. to sharply release heat (see the dotted line forDTA), and the resulting thermal stress generated the fissure in thefiring process. However, there was found an advantage that when PET,PMMA, phenolic resin, crosslinked polystyrene, or the like was used as apore forming agent, the heat flow in the concerned temperature range wassuppressed to such a low level (see the solid line for DTA) that thefissure was very scarcely generated in the firing process.

FIG. 6 is a graph showing the relationships between the soot depositiontime and the pressure loss.

In FIG. 6, the solid line shows the result for the honeycomb structurebody of Example 1, and the broken line in FIG. 4 shows the result forthe honeycomb structure body of Comparative Example 5.

The conditions for the soot deposition were that a light oil gas burnerwas used to generate a gas having a temperature of about 150° C., andthe gas thus generated was made to flow into the DPFs made of thehoneycomb structure bodies of Example 1 and Comparative Example 5 at agas flow rate set of 2.4 Nm³/min.

As can be seen from the results shown in FIG. 6, for the honeycombstructure body of Example 1 in which the porosity was 63% and the arearatio was as large as 45%, the pressure loss rise did not become largeeven after a prescribed elapsed time, whereas for the honeycombstructure body of Comparative Example 5 in which the porosity was 50%and the area ratio was as small as 20%, the pressure loss rise becamelarge with elapsing time.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, it is possibleto provide a honeycomb ceramics structure body which can attain a lowpressure loss and a high collection efficiency, and a method forproducing the same.

What is claimed is:
 1. A honeycomb ceramics structure body havingchemical composition of 42 to 56 wt % of SiO₂, 30 to 45 wt % of Al₂O₃and 12 to 16 wt % of MgO, and a crystalline phase mainly composed ofcordierite, wherein said honeycomb ceramics structure body has aporosity of 55 to 65%, an average pore size of 15 to 30 μm; and a totalarea of the pores exposed on surfaces of partition walls constitutingthe honeycomb ceramics structure body being 35% or more of the totalarea of the partition wall surfaces.
 2. The honeycomb ceramics structurebody according to claim 1 in which the total area of the pores exposedon the surface of the partition wall is 40% or more of the total area ofsaid partition wall surface.
 3. The honeycomb ceramics structure bodyaccording to claim 1 in which the average pore size is from 15 to 25 μm.4. The honeycomb ceramics structure body according to claim 1 in which athickness of the partition wall is 300 μm or less.
 5. The honeycombceramics structure body according to claim 1 in which the permeabilityis from 1.5 to 6 μm².
 6. The honeycomb ceramics structure body accordingto claim 1 in which the coefficient of thermal expansion between 40 and800° C. is 0.5×10⁻⁶/° C. or less.
 7. The honeycomb ceramics structurebody according to claim 1, which can be used as a diesel particulatefilter.
 8. A method for producing a honeycomb ceramics structure bodyhaving chemical composition of 42 to 56 wt % of SiO₂, 30 to 45 wt % ofAl₂O₃ and 12 to 16 wt % of MgO, a crystalline phase mainly composed ofcordierite, a porosity of 55 to 65%, an average pore size of 15 to 30μm; and a total area of a pores exposed on surfaces of partition wallsconstituting the honeycomb ceramics structure body being 35% or more ofthe total area of the partition wall surfaces, wherein 15 to 25 wt % ofgraphite and 5 to 15 wt % of a synthetic resin are added as a poreforming agent to a cordierite-forming raw material, a resultant iskneaded and molded into a honeycomb shape, and thus formed body is driedand fired to produce a honeycomb ceramics structure body.
 9. The methodfor producing a honeycomb ceramics structure body according to claim 8wherein the synthetic resin is any one of poly(ethylene terephthalate)(PET), poly(methyl methacrylate) (PMMA), crosslinked polystyrene, andphenolic resin, or a combination thereof.
 10. The method for producing ahoneycomb ceramics structure body according to claim 8, wherein [theaverage particle size of the raw material talc is 50 μm or less, and theaverage particle size of the raw material silica is 60 μm or less, in]the cordierite-forming raw material comprises talc having an averageparticle size of 50 μm or less and silica having an average particlesize of 60 μm or less.